The photolithography industry is developing Extreme Ultra-Violet (EUV) photolithography systems that operate at 13.5 nm wavelength. Due to the extremely short wavelength used, reflective optical components must be used in the implementation of these systems. Additionally, the surface of these reflective components must be extremely well controlled during manufacture of the components and during use in the end photolithographic application. One such critical reflective optical component is the photomask or reticle that contains the pattern to be lithographically printed on a silicon wafer. In an EUV photolithography system, the reticle is held by an electrostatic clamp. The clamp or chuck includes a generally planar or flat gripping surface referred to as a platen on which the reticle is seated and held in place by electrostatic forces. The reticle must be kept extremely flat during use, and as a result, the electrostatic chuck that holds the reticle must also be held to extremely tight flatness specifications.
A typical electrostatic chuck or clamp 500 is illustrated in FIG. 5. The mounting surface or platen 502 is provided with electrodes 506 for supplying currents necessary for electrostatic attraction. Chuck 500 includes a base 504 supporting platen 502 with electrodes 506 sealed between a base 504 and platen 502. The combination of platen 502, electrodes 506 and base 504 are generically referred to as an electrostatic chuck 500. A heater 508 can be embedded in base 504. Chuck 500 is fixed onto a backing plate 520 typically formed of a high thermally conducting metal. Backing plate 520 can include cooling water channels 522 for controlling temperature.
Recent reticle electrostatic clamp development activities by photolithographic system manufacturers have generated designs that use polymer coatings to help manage the electrostatic fields that are used to hold the reticle on the chuck. These polymers, while delivering the required electrostatic properties, induce bending in the chuck due to shrinkage during cross-linking thus making the chucks unsuitable for use in maintaining the flatness of the reticle during use.